[stubs] Use a single slot for context globals.

[stubs] Use a single slot for context globals.

Don't use different read/write slots for context globals, but
let them share the same slot, which reduces the number of
initial misses, and also saves some memory for large scripts.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1258213002

Cr-Commit-Position: refs/heads/master@{#29889}